Current discharge requirements for treated wastewater from sewage treatment plants generally call for a degree of suspended solids removal unattainable by secondary treatment technology on a predictable and uniform basis. For example, the standards currently applicable in Ohio are 8 mg/L monthly B.O.D. (Biochemical Oxygen Demand) average and 12 mg/L B.O.D. maximum weekly average. Various types of tertiary treatment techniques and devices have been employed, such as slow sand filtration, rapid gravity sand filtration, spray irrigation and polishing lagoons. Prior art patents have disclosed various tertiary treatment equipment and methods, as for example those disclosed in U.S. Pat. No. 4,190,543, U.S. Pat. No. 3,925,205 and in U.S. Pat. No. 3,774,770.
In addition to the removal of these suspended solids it is desirable to remove other contaminants which also exist in the effluent from a secondary treatment. It has been recognized that in the treatment of wastewater a process can be used in which various contaminants are decomposed by microorganisms. More particularly, it is known that when organic substances are subjected to various conditions insofar as temperature and oxygen are concerned that certain desirable aerobic microorganisms are created which decompose certain organic substances and convert them into non-harmful materials such as carbon dioxide, water and the like. In this connection U.S. Pat. No. 4,045,344 discloses a method wherein a bundle of straight tubes or the like are used in what is called a "submerged packing process." In this method the packings are submerged in wastewater in a treating tank and the wastewater is circulated through the packings under aeration so that aerobic microorganisms are generated on the surface of the individual packings aerobic as films. These films decompose various organic substances coming into contact with the films. The patent describes the importance of maintaining turbulent flow through the entire length of the packings and states that turbulent flow is preferred for the propagation of the aerobic microorganisms (see column 4 lines 38-60).
In U.S. Pat. No. 4,190,543, incorporated by reference herein, there is disclosed a tertiary treatment method for removing suspended solids which has proven to be commercially successful. In this process wedge wire media panels are positioned within the treatment tank and are inclined with respect to the vertical axis of the sidewalls. In operation a biological mat is established on the media panels. Canadian Pat. No. 881,668 and U.S. Pat. No. 3,774,770 disclose waste treatment apparatus wherein wedge wire media panels are also utilized.
The principle objectives of the present biofiltration invention have been to provide a tertiary treatment apparatus which provides for a high degree of suspended solids removal and in addition a high degree of carbonaceous B.0.D. and ammonia nitrogen removal. It is a further objective of this invention to provide such a treatment facility wherein continuous operation can be obtained over a wide range of conditions and when cleaning is required sludge buildup may be easily removed from the system.